การแฮชในบล็อกเชน

A hash function takes any input and produces a fixed-size output called a digest or hash.

Blockchains rely on cryptographic hash functions like SHA-256 (Bitcoin) and Keccak-256 (Ethereum).

Hashing is deterministic: the same input always yields the same output.

This lets any node independently verify a hash and arrive at the identical result — essential for distributed agreement.

A tiny change in the input produces a completely different hash. This is the avalanche effect.

Notice how changing one letter changes the entire digest:

No matter how large the input, the output is always the same length. SHA-256 always returns 256 bits (64 hex characters).

This makes hashes convenient, compact identifiers for blocks, transactions, and state.

A good hash function is one-way: given a hash, you cannot feasibly find the input that produced it.

This preimage resistance is why hashes can safely commit to data without revealing it.

Collision resistance means it is infeasible to find two different inputs with the same hash.

If collisions were easy, an attacker could swap one block or transaction for another that shares the same hash — destroying the chain's integrity.

Recall that each block stores the hash of the previous block. Because hashing is deterministic and collision-resistant, this link is secure.

Altering an old block changes its hash, breaking the chain at that point.

Immutability emerges from chained hashes. To change block 100, you must recompute block 100's hash, then 101, 102, and every block after it.

In a proof-of-work chain, that also means redoing all of their mining — practically impossible.

A hash pointer is a reference to data plus the hash of that data. If the data is tampered with, the stored hash no longer matches.

Blockchains are essentially linked lists built from hash pointers.

Hashing also generates account addresses. An Ethereum address is derived by hashing the public key with Keccak-256 and keeping the last 20 bytes.

So hashing is woven through identity as well as integrity.

Cryptographic hashing gives blockchains their core guarantees: deterministic verification, tamper-evidence, compact identifiers, and immutability.

Every block link, Merkle root, and address depends on it.

You learned that:

• Hash functions are deterministic and produce fixed-size output
• The avalanche effect means tiny changes alter the whole hash
• Preimage and collision resistance secure the chain
• Chained hashes create immutability

Next: how this data is shared across a distributed ledger.